Emissions question (gasoline engines) - curiosity got me

[kurek]

First I don't want this to be about whether emissions systems are good or bad, I'm not going to be modifying anything, everyone has their opinions and none of that is why I'm asking this.

I just recently picked up an old (1994) project car to restore and was reminded of the fact that cars used to have smog pumps (secondary air injection) to give their catalytic converters some fresh oxygen to chew on during warm-up. I remember from working on my sister's E46 that it has an electric pump instead of the belt driven one on my project car. But I don't think my 2015 Jeep Patriot or my 2015 Ram 1500 Hemi have any kind of secondary air injection system. Do they?

How do modern engines keep the catalytic converters happy, are new converters made from different catalysts that don't need it? Do modern engines do valve timing tricks to sneak fresh air into the exhaust while the spark plug isn't looking?

Thanks for any constructive conversation on the topic.

 

[EdGs]

That's an interesting question. I had a 2001 Saturn SL that had an air pump on it. Following..

 

[hemihustlin]

if I remember correctly my book learning days,
after a good romp on the throttle when you let off the cats start storing oxygen. if looking at o2 sensor graph it will be flatlining while it is storing.

 

[kurek]

if I remember correctly my book learning days,
after a good romp on the throttle when you let off the cats start storing oxygen. if looking at o2 sensor graph it will be flatlining while it is storing.

That part makes sense, I understand that part of the catalyst coating is a reactive metal which stores and releases oxygen atoms.

Hard for me to picture in my head how much, how fast, how that would all balance out.. like we're talking about a layer of oxygen atoms one thick (presumably) over a few dozen square feet of substrate vs hundreds of cubic feet of exhaust containing trillions of atoms, it just seems like shoveling sand against the tide with a teaspoon. But I didn't go to school for chemistry so all that information is abstract to me, maybe a few oxygen atoms are all it takes?

So if you could help me understand, do I have this right?

when the vehicle is coasting off-throttle it's still pumping air, at least a little bit through the idle bleed/IAC, but the injectors are shut off - the coating on the catalyst is storing up all of that free oxygen and then uses it to help reheat the catalyst when there's extra hydrocarbon (fuel) coming down the line, and then once the catalyst has reached thermal equilibrium it no longer needs additional oxygen to continue functioning on-throttle ?

 

[hemihustlin]

when the vehicle is coasting off-throttle it's still pumping air, at least a little bit through the idle bleed/IAC, but the injectors are shut off - the coating on the catalyst is storing up all of that free oxygen and then uses it to help reheat the catalyst when there's extra hydrocarbon (fuel) coming down the line, and then once the catalyst has reached thermal equilibrium it no longer needs additional oxygen to continue functioning on-throttle ?

yes when the exhaust is lean the cats are storing o2 for when it needs it, to reduce CO and HCs to less harmful products.
Now I didn't have the answer to what happens when you are still on throttle and the cats run out of o2 so I did some research. Here is what I found:

The converter has an efficiency rating that is computed by the vehicle. This number rates the amount of reduction that is occurring in the converter and its ability to store oxygen. But, the efficiency of the converter is tied to the fuel trim of the engine. Most engines minutely alter the fuel trim to replenish the oxygen in the converter and to add fuel for reduction. This helps to keep the converter at the correct temperature for the most efficient operation.

Catalytic Converter Efficiency | Know Your Parts

Almost every part on the engine determines how long a catalytic converter will last. Learn more about catalytic converter efficiency.

www.knowyourparts.com

 

[kurek]

Man now I'm glad I asked, think I might have learned something new.

I always thought the downstream O2 sensor's sole job was to report failure of the catalytic converter (to prompt replacement by the vehicle owner in order to continue being able to legally register the vehicle) and nothing else.. but it sounds like the PCM might also use the downstream data along with upstream to coordinate enrichment/leaning and manage catalyst temperature.

That's cool engineering!

 

[HEMIMANN]

For a spark-ignited engine, wouldn't the exhaust only be lean with a GDI injector capable of lean burn operation? Otherwise it's gotta be stoichiometric at all times for port injection or carburetion, right?

 

[Fast69Mopar]

Man now I'm glad I asked, think I might have learned something new.

I always thought the downstream O2 sensor's sole job was to report failure of the catalytic converter (to prompt replacement by the vehicle owner in order to continue being able to legally register the vehicle) and nothing else.. but it sounds like the PCM might also use the downstream data along with upstream to coordinate enrichment/leaning and manage catalyst temperature.

That's cool engineering!

You are correct about the downstream oxygen sensors. Their main purpose is to monitor catalyst efficiency but they are also used to adjust the LTFT or Long Term Fuel Trims. When the upstream oxygen sensors are lean the STFT or Short Term Fuel Trim start adding fuel and it shows as a positive number. Whatever that positive number is, call it +6, the LTFT correction will be -6 to bring number back as close to zero as possible.

Whatever the STFT number is, the LTFT number will be opposite. When you are accelerating or at WOT (Wide Open Throttle)
the PCM will add fuel above stoich so the engine does not run lean and then the PCM will taper off the extra fuel based on demand and emissions.